A variety of voltage detecting devices have been employed to detect the voltage of a predetermined part of an object such as an electrical circuit. In one example of a voltage detecting device of this type, a probe is brought into contact with a predetermined part of the object to detect the voltage thereof. In another example, the probe is held away from the predetermined part of the object and an electron beam is applied to the predetermined part to detect the voltage thereof.
There has been a strong demand for the provision of a method of detecting the voltage of a part or all of a small object, such as a small integrated circuit, with high accuracy but without affecting the condition of the minute object. A voltage detecting device that uses a contact probe is disadvantageous for this purpose for several reasons. First, it is rather difficult to bring the probe into contact with a part of a small object, e.g., an integrated circuit or the like. Moreover, even if it were possible to do so, it would be difficult to analyze the operation of the integrated circuit correctly only from voltage data obtained in this way. Furthermore, the operating conditions of the integrated circuit would be changed by contact with the probe.
The voltage detecting device that uses an electron beam can detect a voltage when the probe is held away from an object under measurement, but it also suffers from several problems. For example, the part to be measured must be exposed in a vacuum, said also the part may be damaged by the electron beam.
Furthermore, in the conventional voltage detecting devices, the operation speed of the detector has not been able to follow a voltage that changes rapidly. Therefore, conventional voltage detecting devices are further disadvantageous in the instance of a voltage that is quickly changing, for instance in integrated circuits.
In order to solve the above-described problems, a voltage detecting device operating on the fact that the polarization of a light beam is changed by the voltage of a predetermined part of an object under measurement has been described in unpublished Japanese patent application number 137317/87 filed by the present applicant on May 30, 1987. FIG. 8 is a diagram showing the arrangement of the that voltage detecting device.
As shown in FIG. 8, the voltage detecting device 50 comprises an optical probe 52, a CW light source 53 (including for instance a laser diode), an optical fiber 51 for guiding a light beam from the light source 53 through a condenser lens 60 to the optical probe 52, an optical fiber 92 for conducting a reference light beam from the optical probe 52 through a collimator 90 to a photo-electric conversion element 55, an optical fiber 93 for applying an emergent light beam from the optical probe 52 through a collimator 91 to a photo-electric conversion element 58, and a comparison circuit 61 in which the output electrical signal of the photo-electric conversion elements 55 and 58 are subject to comparison.
Electro-optical material 62 such as an optically uniaxial crystal of lithium tantalate (LiTaO.sub.3) is enclosed in the optical probe 52. The end portion 63 of the optical probe 52 is in the form of a circular truncated cone. A conductive electrode 64 is formed on the cylindrical wall of the optical probe 52. A reflecting mirror 65 made of a metal film or dielectric multi-layer film is provided on the end face of the end portion 63 of the optical probe 52.
Provided in the optical probe 52 are a collimator 94, condenser lenses 95 and 96, a polarizer 54 for extracting only a light beam having a predetermined polarization component out of a light beam outputted by the collimator 94, and a beam splitter 56. The beam splitter 56 divides the light beam having the predetermined polarization component, which is provided by the polarizer 54, into a reference light beam and an incident light beam, and applies an emergent light beam from the electrooptical material 62 to an analyzer 57. The reference light beam and the emergent light beam are applied to the optical fibers 92 and 93 through the condenser lenses 95 and 96, respectively.
In performing voltage detection with the voltage detecting device thus organized, the conductive electrode 64 formed on the cylindrical wall of the optical probe 52 is normally grounded. Under this condition, the head 63 of the optical probe 52 is set close to an object under measurement, such as an area of an integrated circuit (not shown). As a result, the refractive index of the head 63 of the electro-optical material 62 in the optical probe 52 will be changed. More specifically, in an optically uniaxial crystal, the difference between the refractive index of an ordinary ray beam and that of an extraordinary ray beam in a plane perpendicular to the optical axis is changed.
The output light beam of the light source 53 is applied through the condenser lens 60, the optical fiber 51 and the collimator 94 to the polarizer 54, which provides a light beam having the predetermined polarization component and being an intensity equal to I. The output light beam of the polarizer 54 is applied through the beam splitter 56 to the electro-optical material 62 in the optical probe 52. The reference light beam and the incident light beam divided by the beam splitter 56 are I/2 in intensity. As was described above, the refractive index of the electro-optical material 62 is changed by the voltage of an object under measurement. Therefore, the incident light beam applied to the electro-optical material 62 is changed in polarization at the head 63 depending on the change in refractive index, and is then reflected by the reflecting mirror 65, so that it is applied, as the emergent light beam from the electro-optical material 62, to the beam splitter 56. The polarization of the incident light beam is changed in proportion both to the difference in refractive index between the ordinary ray and the extraordinary ray due to the presence of a voltage, and to a value 21 where 1 is the length of the head 63 of the electro-optical material 62.
The emergent light beam is applied to the analyzer 57 by the beam splitter 56. The intensity of the emergent light beam applied to the analyzer 57 is reduced to I/4 by the beam splitter 56. If the analyzer 57 is designed to transmit only a light beam having a polarization component perpendicular to the polarization component of the polarizer 54, then the intensity I/4 of the emergent light beam applied to the analyzer 57 is changed to (I/4)sin.sup.2 ((.pi./2).multidot.V/V.sub.O) by the analyzer 57 before being applied to the photo-electric conversion element 58,V is the voltage of the object under measurement, and VO is the half-wavelength voltage.
In the comparison circuit 61, the intensity I/2 of the reference light beam subject to photo-electric conversion by the photo-electric conversion element 55 is compared with the intensity (I/4)sin.sup.2 ((.pi./2).multidot.V/V.sub.O) of the emergent light beam subjected to photo-electric conversion by the photo-electric conversion element 58.
The intensity (I/4) sin.sup.2 ((.pi./2).multidot.V/V.sub.O) of the emergent light beam depends on the change in refractive index of the head 63 of the electro-optical material 62 which is due to the variation of voltage. Therefore, the voltage of a predetermined part of an object under measurement, such as an area of an integrated circuit, can be detected from the light intensity.
As was described above, the voltage detecting device 50 of FIG. 8 is designed so that the voltage of a predetermined part of an object under measurement is detected from the change in refractive index of the end portion 63 of the electro-optical material 62 which is caused when the end portion 63 of the optical probe 52 approaches the object. Therefore, with the device, the voltage of a small part of an integrated circuit which is difficult for the probe to contact or is affected in the voltage when contacted by the probe can be detected when the optical probe 52 is held a small distance away from the area to be measured. Furthermore, a pulse light source such as a laser diode that outputs an optical pulse with an extremely short pulse width may be employed to sample quick voltage changes of an object which occur at considerably short time intervals. A CW light source and a high-speed response detector such as a streak camera, may be used to measure the rapid voltage change of the object with high time resolution. In that case, the quick voltage change can be detected with high accuracy.
However, the voltage detecting device 50 of FIG. 8 suffers from some difficulties. The device detects the voltage of a predetermined part of an object under measurement from the variation in polarization of the light beam in the electro-optical material 62. Therefore, it is necessary to extract only the light beam having the predetermined polarization component from the output light beam of the light source 53 with the aid of the polarizer 54 and to extract the predetermined linear polarization component from the emergent light beam from the electro-optical material 62 with the aid of the analyzer 57. Consequently, the device suffers from low utilization. Furthermore, the device uses the beam splitter 56 which causes the intensity of the emergent light beam applied to the analyzer 57 to be lower than that of the light beam outputted by the light source 53. As a result, the voltage detection accuracy of the detector is unduly limited. Moreover, the device requires a large number of components, such as the polarizer 54, the analyzer 57, and the beam splitter 56, which causes the improvement in accuracy of the optical system to be limited.
If instead of the photo-electric conversion elements, a streak camera is employed as the detector, the variation of voltage of the predetermined part of the object under measurement is detected in the form of a one-dimensional optical intensity distribution on the phosphor screen of the streak camera. In order to obtain the waveform of the voltage, it is necessary to analyze the one-dimensional optical intensity distribution on the phosphor screen of the streak camera. In addition, in the method of detecting of the voltage of a predetermined part of an object under measurement from the change of polarization, only the absolute value of voltage is detected, and it is impossible to detect the polarity of the voltage. In other words, it is impossible to determine whether the voltage is positive or negative.